Historically, hydraulic pressure devices have pressurized the entire internal cavity of the pressure device. This necessitates the use of high pressure seals throughout the device including surrounding the input/output shaft. In addition, it subjects the hydraulic fluid to contamination based on the physical contaminants produced by the wear between the input/output shaft to the power mechanism such as to and from the wobble stick in a gerotor device. U.S. Pat. Nos. 3,606,601 and 4,697,997 are examples of typical gerotor hydraulic motors. These gerotor hydraulic motors are serviceable. The motors do, however, both use a pressurized center. Closed center gerotor motors have many advantages, a number of which are described in Hollis White's seminal U.S. Pat. No. 4,877,383 (Device Having A Sealed Control Opening And An Orbiting Valve). U.S. Pat. No. 4,877,383 issued Oct. 31, 1989 provides a solution to the pressurized fluid problem by effectively isolating the center of the gerotor device from the operative fluid to and from the fluid power mechanism. The device of U.S. Pat. No. 4,877,383 does, however, show in its preferred embodiment the use of seals on the inside of the orbiting valve in order to seal the connection to the wobble stick drive. These seals necessitate a somewhat larger inner section for the valve than would be otherwise possible. The seals also serve to restrict the size of the wobble stick drive interconnection to the orbiting valve. In addition, the drain for the center of the hydraulic device is via a separate drain line located outside of the gerotor structure. These provide limits for the application of the preferred embodiment of U.S. Pat. No. 4,877,383.
This present invention is directed to an improvement to the closed center type devices wherein the seal for the central drive opening of the commutator valve is provided through the use of a moveable piston. This seals the opening while also providing additional advantages including the ability to centralize and simplify the case drain for the device.